Impact of RDH Extended Delignification on ECF and TCF Bleaching, 1994 Pulping Conference Proceedings

Nam Hee Shin, Fernando Mera

The RDH-Kraft pulping process has demonstrated the ability to cook softwood pulps to 12 kappa number without loosing selectivity (yield vs. kappa). Pulps at kappa number range of 12-22 were bleached using the ECF (Elemental Chlorine Free) and TCF (Total Chlorine Free) bleaching sequences to +90% ISO brightness.

Softwood pulps cooked to lower kappa number, bleached by ECF and TCF bleaching sequences, have better bleachability than higher kappa number pulps. The bleaching of RDH-kraft pulp using ECF and TCF sequences demonstrates that extended delignification to lower kappa number is a key factor to increase final brightness. Full market pulp brightness (+90% ISO) can be achieved using TCF sequence without ozone when lower kappa number oxygen delignified pulp is bleached at high pulp consistency in the (P)-stage. Bleached pulp viscosity does not correlate with tear index. However, it has a linear correlation with tensile index. Higher tensile index is obtained at higher viscosity. In the TCF bleaching sequence, there is no correlation between COD values and kappa number. Lower amounts of COD were generated from ECF bleaching sequence than TCF sequences.